![]() ![]() As more air bubbles up, those bubbles are also trapped inside the ever-thickening ice underneath the first layer of bubbles. As the cooling from the air above continues, the ice continues growing downward into the water and forms around the bubbles, until the ice is eventually thick enough to fully enclose the bubbles, one by one. Once there is a thin layer of ice covering the lake, the air bubbles cannot escape from the water and are trapped below the ice. How did those air bubbles get there? The bottom of the lake is muddy and sometimes air bubbles come out of the mud and bubble up through the water, all the way to the surface. Figure 2 shows the frozen surface of a lake, with air bubbles inside the ice. Sometimes we can even see that ice forms from the top down. Therefore, cooling, and thus freezing, can only happen from top down. Below and to the sides, the water is still surrounded by more water or the by sea/lake floor, which are warmer than the air. However, when water freezes in a lake or the ocean, the cold air is only above the water’s surface. When ice forms in the freezer, the ice cube tray is surrounded by cold air on all sides. But there are a couple of reasons why it is a lot more difficult for the ocean to freeze all the way through than for an ice cube to do so-and that is not just because there is more water in the ocean than in the freezer. If it was cold enough for a long enough time, the whole ocean might freeze all the way through. ![]() Top-Down Business: Freezing a Lake or the Seaīut wait, why is there water underneath the ice? Why does the ice not form throughout the whole depth of the water, as it does with ice cubes in the freezer? Well, in a way, the ice formation in the sea is not finished yet. Over time, smaller pancakes freeze together to form larger and larger pancakes, and, eventually, they can freeze together to cover large areas of the ocean ( Figure 1D). The edges of the ice floes get rounded off by these collisions, and the bits that break off get pushed on top of the round ice floes, giving the ice pancakes rims around their edges. As they are moved by waves, the ice floes bump into each other. Pancake ice forms when ice needles freeze together to form larger ice floes. Usually, there are waves during ice formation, and you see pancake ice. After this thin layer forms, it can break up into several ice floes. Initially, this layer is transparent enough to see through and spot algae growing on the sandy bottom underneath ( Figure 1B). Sometimes, on a calm day when there are no waves disturbing the process, needles can freeze together and a layer of ice forms. The surface of the sea ice is covered in snow (Photos: Mirjam S. (D) Ice-covered ocean with a track created by a ship moving through. (B) A thin layer of ice that was broken into larger ice floes after freezing. (A) Open water (upper left), ice slush (middle), ice pancakes (lower right).Figure 1 - Stages of sea ice formation.The slush slightly calms the waves coming in from the open water, but there are still enough waves to prevent the needles from freezing together and forming a smooth ice surface. Then, there is a zone where ice has started to form in tiny needle-shaped crystals that come together to form ice slush ( Figure 1A, in the middle). When ice forms in the sea, it therefore looks different from ice formed in your freezer, and sea ice forms through several different stages.įirst, there is a stage when the water is very cold, but still liquid (see the top left corner of Figure 1A). In the sea, there are almost always disturbances occurring during ice formation, because there are almost always waves. Keeping it Moving: Waves Disturb Sea Ice Formation There might be layers going through the ice that are not transparent, and the surface might be less smooth. This means that it will not look as clear, because not all of the molecules were well-organized as they froze. This means the water molecules froze in an organized crystal structure.īut what if someone opened the freezer? If ice is disturbed while growing, it forms irregularities. If nobody opened the freezer after you put the tray of water inside, the ice cubes will have frozen slowly, to look like clear glass. Get an ice cube tray, fill it with water, put it in the freezer, and come back the next day. If you have a freezer in your kitchen, you can easily investigate ice formation for yourself. But have you ever wondered how exactly ice forms and what determines what it looks like in the end? In this article, we investigate how the formation of ice is influenced by the movement of water while it is freezing, by the direction the cooling is coming from, and by salt dissolved in the water. ![]() We see ice all the time: as cubes in drinks, forming on pools of rainwater in winter, in movies like Frozen or The Blue Planet. ![]()
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